An ignition plug used for an internal combustion engine of, for example, of a vehicle includes a center electrode, and a ground electrode which has a bent portion and is disposed to face the center electrode. In order to enhance the ignition performance of such an ignition plug, there has been proposed a method of generating plasma on the basis of spark discharge (trigger discharge) to thereby expand flame (See WO2012/111700 A2, WO2012/111701 A2 and Japanese Patent Application Laid-Open (kokai) No. 2010-102864; hereinafter respectively referred to as Patent Documents 1 through 3).
However, in the case of the ignition plugs disclosed in Patent Documents 1 and 2, trigger discharge occurs not only at the distal end of the ground electrode but also at a position near a bent portion of the ground electrode at a high ratio. Therefore, these ignition plugs have a problem in that, when trigger discharge occurs at a position near the bent portion of the ground electrode, the growth of plasma is hindered by the ground electrode, and expansion of flame is restricted. Also, the ignition plug disclosed in Patent Document 3 has a problem in that, since trigger discharge occurs, in a concentrated manner, on a surface of the ground electrode which faces the center electrode, the ground electrode consumes locally, and the overall spark-consumption resistance of the ground electrode deteriorates.